A circuit pattern, which is composed of electrodes on which various electronic parts are mounted and resistors, has been generally formed on a ceramic base made of alumina, zirconia, etc. and the electrodes are generally formed by screen printing a conductive paste containing a noble metal, e.g., Ag or Ag--Pd, on the ceramic base followed by baking in air. Because of its expensiveness, the noble metallic paste has recently been displaced with a conductive paste containing a base metal, e.g., Cu, Ni or Al. In using such a base metallic paste, the screen-printed paste on a ceramic base is generally baked in a neutral or reducing atmosphere. If the base metallic paste is heated at high temperatures (i.e., baked) in an oxygen-containing atmosphere, such as air, a metal oxide would be formed which is an insulator. This is the reason why baking of a base metallic paste should be conducted in a neutral or reducing atmosphere.
When electrodes are formed by using such a base metallic paste, resistors which are arranged to bridge over the electrodes should also be formed by using a resistive paste which can be baked in a neutral or reducing atmosphere. Known resistive pastes which can be baked in a neutral or reducing atmosphere include LaB.sub.6 -based pastes, NbB.sub.2 -based pastes, and the resistive paste proposed by the present inventors and disclosed in U.S. Pat. No. 5,036,027. The paste described in U.S. Pat. No. 5,036,027 comprises a mixture of a resistive material having a composition of Nb.sub.x La.sub.1-x B.sub.6-4x (wherein x is from 0.1 to 0.9 mol) and non-reducing glass frit, kneaded with an organic vehicle.
A desired surface resistivity over a broad range has been obtained by varying the mixing ratio of these resistive materials and glass frit. However, in using the LaB.sub.6 -based or NbB.sub.2 -based resistive pastes, the surface resistivity suffers drastic changes with a slight variation in glass frit amount, and satisfactory reproducibility cannot be assured. On the other hand, resistors formed of the Nb.sub.2 La.sub.1-x B.sub.6-4x -based paste show a milder increase in surface resistivity than with those formed of the LaB.sub.6 -based pastes and NbB.sub.2 -based pastes. Therefore, the Nb.sub.x La.sub.1-x B.sub.6-4x -based paste has an advantage of a broadened surface resistivity range of from 10 .OMEGA./square to 10 M.OMEGA./square by varying the mixing ratio of resistive material to glass frit. However, the resistors formed of the Nb.sub.x La.sub.1-x B.sub.6-4x -based paste, particularly those adjusted to have a high surface resistivity (e.g., from 1 k.OMEGA./square to 10 M.OMEGA./square), sometimes undergo deterioration in temperature coefficient of resistivity (hereinafter abbreviated as "TCR"), i.e., shift of TCR in the minus (-) direction, and do not always satisfy the characteristics required for practical use.